The firing sequence in a drivage is shown. The charge per hole for Sections 1, 2, and 3 are 2.0 kg, 3.0 kg, and 1.0 kg, respectively, and the delay between each section is 25 milliseconds. The maximum charge per delay, in \(kg\), is
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Work out the total charge fired in each section, then check whether the 25 ms gap between sections is enough to keep them on separate delays instead of adding together.
Step 1: Read the blast layout.
The figure shows three concentric rings of blastholes drilled in the face of a drivage, fired in sequence. The innermost diamond of holes is Section 1, the next ring out is Section 2, and the outer ring closest to the drive boundary is Section 3. Counting the dots in the figure, Section 1 has 4 holes, Section 2 has 4 holes, and Section 3 has 8 holes.
Step 2: Find the total charge fired in each section.
Each hole in a section carries the same charge, and every hole in one section fires together on the same delay number, so the total charge of a section is its hole count times its charge per hole.
Section 1: \( 4 \times 2.0 = 8.0 \) kg
Section 2: \( 4 \times 3.0 = 12.0 \) kg
Section 3: \( 8 \times 1.0 = 8.0 \) kg
Step 3: Decide whether charges from different sections add together.
For ground vibration purposes, charges that fire within about 8 milliseconds of one another behave as one combined instantaneous charge, since the ground and the seismograph cannot separate blasts spaced that close together. Here the gap between sections is 25 milliseconds, well above that 8 millisecond limit, so Sections 1, 2 and 3 fire as three separate, independent delays. Their charges are never added across sections.
Step 4: Pick the maximum charge per delay.
The maximum charge per delay (often written MCD) is simply the largest total charge that goes off on any single delay. Comparing the three section totals, 8.0 kg, 12.0 kg and 8.0 kg, the biggest is Section 2 at 12.0 kg.
\[ \boxed{12.0 \text{ kg}} \]
